Installation guide

48 Planning and Installation Guide ShoreTel 14.2
Network Requirements and Preparation Packet Loss
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The jitter buffer starts at the minimum size of 0 ms as packets from the network are placed into the
switchboard queue for immediate processing. When jitter is detected on the network, the jitter buffer
dynamically increases in increments of 5 ms to compensate for increased jitter and decreases in
reaction to less jitter. The maximum value of the jitter buffer is set in ShoreTel Director and ranges from
20 to 400 ms, with a default of 300 ms.
As the jitter increases on the network and the jitter buffer needs to be increased to guarantee timely
audio play, the latency of the audio also increases. The system attempts both to maintain a minimum
jitter buffer size that provides good-quality voice without dropping packets and to provide minimum
latency.
For third-party IP phones that are configured in the ShoreTel system, the jitter buffer is not
configurable. The minimum jitter buffer is 10 ms, and the maximum is 80 ms.
Maximum jitter buffer values greater than 100 ms should rarely be necessary. If needed, this could
indicate a problem in your network that should be addressed in another way. As the jitter buffer depth
increases, the latency experienced by the user increases. For this reason, network jitter should be kept
to a minimum.
Packet Loss
Lost packets can occur on the IP network for any number of reasons. Packet loss above 1% begins to
adversely affect voice quality. To help reduce this problem, ShoreTel voice switches and 400-Series IP
phones have a feature called lost packet concealment. When a packet is lost on the network, the last
sample received is replayed to the receiving party at a reduced level. This is repeated until a nominal
level is reached, effectively reducing the clicking and popping associated with low levels of packet loss.
Fax and modem calls demand essentially zero packet loss to avoid missing lines on fax calls and to
avoid dropped modem calls. In addition, fax and modem calls, when detected, may change to a
higher-rate codec.
Bandwidth Management
In addition to the network requirements discussed above, bandwidth management techniques need to
be deployed to ensure that real-time voice data is not affected by bursts or high amounts of data traffic.
Local Area Network (LAN)
To manage bandwidth in the local area network (intrasite) and meet the requirements for toll-quality
voice, use Ethernet switching. Ethernet switching is cost effective and simple to provision. Your LAN
configuration requirements will vary depending on your infrastructure and whether your network
includes IP phones.
IP phones sample the user’s voice and convert the voice signal to IP packets using the Real Time
Protocol (RTP). These packets must be tagged for higher prioritization in the network. ShoreTel IP
phones have embedded Ethernet switches and automatically prioritize voice traffic ahead of any data
traffic coming from daisy-chained personal computers (for example, large file transfers and e-mail).